Compact diodes emitting in the ultraviolet (UV) domain recently have matured to be used in industrial, engineering, scientific, and medical applications. These light sources are used for disinfection, polymer curing, and skin illness treatment. In phototherapy, detrimental side effects can be minimized by carrying light through fiber optics so that a small output beam can be targeted at selected areas of the infected skin. Other applications of UV radiation include coherent anti-Stokes Raman scattering and fluorescence imaging.
A commonly utilized transparent material for guiding UV light includes solid-core fibers fabricated based on fused silica. In addition, the use of hollow-core photonic crystal fiber (HC-PCF) allowing weak interaction between the glass material and the light may assist in overcoming some drawbacks presented in solid-core fused silica fibers. For example, in the infrared (IR) region of the electromagnetic spectrum, HC-PCFs have been shown to guide light with loss thirty times lower than that of its glassy constituent. To this extent, an HC-PCF-based design for efficient propagation of UV light at wavelengths of 355 nm has been suggested.
A group considered using photonic crystal fibers (PCFs) for single-mode delivery of UV wavelengths in the range ˜200-300 nm. Typical PCFs have a uniform patterned microstructure of holes (defects) running axially along the fiber channel with a missing hole in the center providing a core region. In an equivalent index-of-refraction picture, the microstructure imposes a strong wavelength dependence on the index-of-refraction of the cladding, and for high light frequencies (short wavelengths) the cladding index approaches the core index. With appropriate fiber design, the fiber core can support a single guided mode over all optical frequencies, a characteristic referred to as endless single-mode operation.
Similarly, a group studied single-mode optical fiber use in high-power, low-loss UV transmission. The group reported large-mode-area solid-core photonic crystal fibers made from fused silica that resist ultraviolet (UV) solarization even at relatively high optical powers. Using a process of hydrogen loading and UV irradiation of the fibers, the group demonstrated stable single-mode transmission over hundreds of hours for fiber output powers of 10 mW at 280 nm and 125 mW at 313 nm (limited only by the available laser power). Fiber attenuation ranges from 0.9 dB/m to 0.13 dB/m at these wavelengths, and was unaffected by bending for radii above 50 mm.
Liquid core waveguide or light guiding structures can be beneficial for guiding ultraviolet (UV) radiation, e.g., due to the low UV absorbance properties of some liquids, such as purified water. The general availability of water allows for the fabrication of relatively inexpensive light guides for UV radiation that can be readily adopted for use in industry. Combined with a light guide enclosure formed of a fluoropolymer having low UV absorbance and other beneficial properties (e.g., chemical inertness, low biological contamination), the benefits of thin light guiding UV layers can be easily appreciated.
These light guiding structures, or so-called liquid core waveguides or flow cells, have been developed for optical spectroscopy applications in the ultraviolet, visible, and infrared regions of the light spectra. Such flow cells are particularly suitable when combined with optical fibers for light transfer, enabling the design of a flexible sensor system. A number of flow cells having a long optical path length have been designed for absorbance, fluorescence, and Raman spectroscopy. Similar to optical fibers, light is confined in such flow cells within the (liquid) core by total internal reflection (TIR) at the liquid core/wall interface or the liquid core/cladding (coating) interface. The only requirement is that the liquid core refractive index be higher than that of the refractive index of the ambient. For liquid core comprising purified water, and for ambient being air, this requirement is easily satisfied.
One approach to employ liquid-based light guiding structures describes a reactor configuration for UV treatment of water utilizing TIR and a flow tube. The inlet and core of the cylindrical tank reactor unit is a transparent flow tube that is surrounded by a sealed, concentric volume of material having a lower refractive index than the fluid flowing in the flow tube, which enables TIR of UV light when it is directed axially into the flow tube. Another approach discloses a method and reactor for in-line treatment of fluids and gases by light radiation comprising a tube or a vessel made of transparent material, preferably quartz glass, and surrounded by air, and having a fluid inlet, a fluid outlet, and at least one opening or window adapted for the transmission of light from an external light source into the tube. Air outside the tube or vessel has a lower refractive index compared to the treated fluid, which enables TIR. Still other approaches discuss various aspects of a liquid core light guide. One such approach discusses a liquid core waveguide photon energy material processing system.